Seriousness:

Mark Manson:

"A man who is very serious and unable to laugh at himself or the world conveys that he is heavily invested in the perceptions of the world around him ..."

Manson wants to point out that it is bad to be invested in the perceptions of another person. I am not so sure about that. E.g. if there is a person you highly respect, you will be invested in his or her perceptions. Status generates nervousness. In particular if you perceive another person high in status and if you care about the outcome of the interaction.